Intensive insulin therapy for Type 1 Diabetes Mellitus (T1DM) is associated with an increased risk of hypoglycemia. Our past NIH-funded research, investigating the biopsychobehavioral causes and effects of hypoglycemia, has yielded important and unique, albeit post hoc, findings about hypoglycemia and driving in T1DM. We demonstrated that mild hypoglycemia (e.g., 65 mg/dl) impairs cognitive-motor, judgment and driving performance. Additionally, our 11-site retrospective survey of 600 drivers with and 300 drivers without diabetes found that those with T1DM had twice as many auto crashes and violations and experienced hypoglycemic stupor 4 times as often while driving than either T2DM or nondiabetic drivers (p's<.01); Post hoc analyses revealed that T1DM drivers who had crashed/hypoglycemic stupor in the previous 2 years had a lower Blood Glucose (BG) threshold for deciding when not to drive, were less likely to measure BG before driving, were more likely to have mild hypoglycemia while driving, and were less likely to have carbohydrates in the car (p's<.03). Finally, our previous research demonstrates that judgment about driving when BG is low can be improved, and number of moving vehicle violations and crashes can be reduced, through psycho-behavioral interventions. Based on these findings, we propose the first, a priori, prospective, multi-center investigation of hypoglycemia and driving in T1DM drivers who are thought to be at High vs. Low risk for driving mishaps. This study has three phases: Phase 1 will involve recruiting 275 High and 275 Low risk T1DM drivers, from three diverse sites in the U.S., and prospectively tracking diabetes self-management behaviors and driving mishaps over a 12-month period. Phase 2 will be intensive and experimental UVA field and hospital-based studies investigating the physiological, behavioral and psychological parameters of driving and self-regulation during euglycemia and hypoglycemia. Phase 3 will include the pilot testing of an intervention aimed at preventing hypoglycemia-related driving mishaps. The proposed research is designed to: A) document the frequency of hypoglycemia-related driving mishaps, e.g., driving errors that may or may not result in a crash or citation; B) determine mechanisms underlying these mishaps, testing a diabetes-specific causal model we have developed; and C) create a database on which to begin to develop an intervention program intended to reduce the factors associated with hypoglycemia-related driving mishaps; and, D) pilot test an intervention designed to reduce risk of driving mishaps due to hypoglycemia. Ultimately, this research may enhance driving safety for people with T1DM.